From a neurobiological point of view, mirtazapine is my number 1 choice due to complicated antagonizing effects on various 5-HT receptors and it's noradrenergic action. What is good with mirtazapine are the cognitive enhancements. Most antidepressants can give you memory trouble etc but mirtazapine can improve memory. Instead of switching medication right now I would try to add Wellbutrin to take in the morning. It's action on dopamine and norepinephrine might be the only thing you need more and especially since mirtazapine is sedative and Wellbutrin is stimulating. Taking both medications will take care of all your monoamines which are thought to contribute to depression.
I would not recommend Effexor. Zoloft is usually good at treating GAD since it's a ''soft'' SSRI but I don't think it will do any better than Celexa.
You can also switch the mirtazapine to a tricyclic antidepressant like nortriptyline is really want to switch the mirtazapine. Nortriptyline is a second generation tricyclic and therefore this is why I would use it first if you really want to try a tricyclic.
Then you have the new Cymbalta which is a potent SNRI you can try but it is extremely hard to quit, just like Effexor so I don't recommend it if I am right on you intentions to get on it for a year or two.
I would really try the mirtazapine before bedtime and Wellbutrin in the morning and see if it works. It's less trouble than to switch medications. Also mirtazapine is extremely safe regarding interactions and serotonin storm.
It's funny you mentioned Wellbutrin. I was given a sample about 10 yrs ago for smoking sessation. I took if for 2 weeks and I remeber feeling kind of happy. It was strange, I woke up one morning and remeber feeling a little happier that I had in a long time. I ran out of the med and went back to smoking and not thougth about it until I started taking AD's. I knew nothing about them back then.